McManus was club captain of Celtic from 2007 until his move to Middlesbrough in 2010. McManus was a product of Celtic's youth academy and had risen through the ranks to become first team captain ahead of the 2007–08 season. Hall of Famer and former Scotland captain Colin Hendry described him as "the last of an ancient breed",[3] in the sense that his no-nonsense defending style is rare in modern football.

McManus came through the ranks at Celtic and signed a professional contract on the same day as fellow defender John Kennedy.[when?] He made his debut for the first team, under Martin O'Neill, in the 2003/04 season against Hibernian, and finished that season with 5 league appearances as Celtic won the league.[4] He later established himself as first choice central defender at Celtic during Gordon Strachan's managerial reign.

In the 2005/06 season, McManus scored eight goals (seven of which came in the League).[citation needed] He was made captain for the first league game of the 2006/07 season, in the match against Kilmarnock due to regular club captain Neil Lennon being suspended.[citation needed] Since that game he repeatedly deputised as Celtic captain throughout the 2006/07 season. After Lennon left Celtic to join Nottingham Forest, McManus was announced as Celtic's new captain on 31 July 2007 and signed a four-year contract. He said:

"To be given the captaincy of Celtic is a tremendous honour. It is undoubtedly one of the highlights of my career so far. I'm proud to follow in the footsteps of so many great Celtic captains from the past and I hope that I will be able to achieve as much success as they have over the years."[5]

McManus was loaned by Celtic to Middlesbrough for the second half of the 2009–10 season.[7] This meant that McManus linked up again with Gordon Strachan, who signed four other Celtic players during the January 2010 transfer window.[7] McManus received the man of the match award in his Middlesbrough debut against Ipswich Town. He scored his first goal for the club in a 2–0 win over Plymouth Argyle on 5 April 2010.[8]

On 13 July 2010, McManus completed a £1.5 million move to the Teesside club signing a 3-year contract.[9][10] McManus scored his second goal for Middlesbrough against Leicester City on 2 April 2011, his goal coming in the 94th minute to level the game at 3–3.[11] At the end of the 2012–13 season, McManus was not offered a new contract and left the club.

McManus signed for Bristol City on loan until the end of the season on 14 February 2012 and made his debut at home in the 2–2 draw against Crystal Palace the same day. He was recalled on 28 March, due to injury worries to Matthew Bates and Seb Hines following the match against Bristol City (McManus did not play against Boro).[12] He then returned to Bristol City for a second loan spell the following season.[13] His first and only goal for the club came in a 4–2 win over Peterborough United on 29 December 2012.[14]

On 15 July 2013, McManus signed a one-year deal with Scottish Premiership side Motherwell.[15] He made his competitive debut for Motherwell in a home Europa League match against Russian side Kuban Krasnodar. Motherwell lost the match 2–0. His first goal for the club came in a 1–0 win over Hibernian on 3 November 2013.[16] McManus made 42 appearances for Motherwell during the 2013–14 season, after which he signed a new two-year contract with the club.[17]

On 14 August 2017, it was announced that McManus had decided to retire from professional football and take up a coaching role with Motherwell.[18]

On 10 September 2008, McManus was sent off for a deliberate handball in a FIFA World Cup qualifier against Iceland at Laugardalsvöllur in Reykjavík.[23] He used his right hand to tip the crossed ball over the crossbar to stop approaching Icelandic striker Heiðar Helguson from scoring a goal, while Scotland were leading 2–0.[23] The penalty kick was converted to make the score 2–1, which was the final score.[23] On 7 September 2010 McManus scored a dramatic 97th minute winning header for Scotland in their 2–1 victory over Liechtenstein.[24] His 26th and final cap came in October 2010, in a 3–2 defeat to Spain.[19]

McManus was born in Hamilton, Lanarkshire. He attended Holy Cross RC Secondary School in Hamilton, the school which international and former club team mate Paul Hartley also attended. McManus and Paul Hartley used to have a competition at training to see who could name the most teachers and the loser picked up all the balls.